Research: Class of 2004

Repetitive masticatory forces can cause the propagation of microfractures and eventual breakdown of the dental materials through the process of fatigue. Orthodontic brackets are subjected to cyclical loading, with the forces transmitted through the bonding material attaching the brackets to the teeth. Brackets made of different materials will have different flexure properties that may influence fatigue properties of the bond. The purpose of this bench-top study was to investigate the fatigue strength of the bond with brackets made of titanium, stainless steel or ceramic material, and to relate the bond's fatigue strength to its shear bond strength. The orthodontic brackets were individually bonded to extracted mandibular premolar teeth using Transbond XT composite (Unitek, Monrovia, CA). For each bracket type, 24 samples were subjected to fatigue tests and 8 to shear bond tests. Fatigue testing employed a "staircase" methodology where samples of each bracket material were tested sequentially by placing a load on the occlusal tie-wings for 5000 cycles. For each bracket material, a predetermined load based on pilot study data was applied for the first trial. If the bracket bond failed before the end of the trial, the load was decreased 5% for the next sample. If there was no failure, the load was increased 5%. The load was recorded and the process continued sequentially until all samples had been tested. For the shear bond strength an increasing load was applied to the occlusal tie-wings until bond failure. The measurements were compared using ANOVA and Tukey's multiple comparisons statistical tests set to p <0.05. Results showed that mean fatigue strengths for the titanium brackets (6.78 +/- standard deviation 0.53 MPa) and stainless steel brackets (5.97 +/- 0.37 MPa) were not significantly different. Bond fatigue strength for the ceramic bracket was significantly higher (9.60 +/- 0.44 MPa) than that of titanium or stainless steel brackets (p< 0.05). The shear bond strength for the titanium bracket (8.66 +/- 1.37 MPa) did not differ from that of the stainless steel bracket (9.43 +/- 1.55 MPa), and was significantly higher for the ceramic bracket (12.06 +/- 2.17 MPa; p < 0.05). The fatigue strength of the bond for the three bracket types was significantly different than the shear bond strength (p< 0.02) where the fatigue strength of the bond for titanium brackets was 78% of the shear bond strength, 63% for stainless steel brackets and 80% for ceramic brackets. Results of this study show that for the three bracket types, the bond strength was lower in fatigue testing than in shear testing. In addition, in both fatigue and shear testing the bond with titanium and stainless steel brackets performed comparably while with ceramic brackets the bond withstood significantly higher forces.

External apical root resorption is a pathologic process associated with orthodontic treatment. The purpose of this study was to evaluate the severity and risk factors associated with root resorption of maxillary central incisors in orthodontic patients who presented with an anterior open bite malocclusion. This retrospective study used randomly selected open bite and control samples, 32 patients in each, screened from 800 orthodontic treatment records at Oregon Health and Science University. The open bite sample was defined as having no vertical overlap of the incisors prior to treatment. The control sample consisted of patients with 10-30% overbite. Subjects were excluded if there was a history of trauma to the maxillary central incisor, existing restoration of the incisal edge, reshaping or restoration of incisal edges during or after orthodontic treatment, root canal therapy, significantly malformed roots, incomplete root apex development, an obscured root apex, greater than 7 mm overjet, supernumerary teeth in anterior maxilla, a history of previous orthodontic treatment (including phase one treatment), orthognathic surgery during treatment, sibling relationship with another subject, or early termination of treatment. Each group contained 9 males and 23 females, with a mean age of 18.7 years for the open bite group and 14.7 years for the control group. Tooth length and changes in tooth position were measured from tracings of the pre- and post-treatment lateral cephalograms. In addition, treatment records were reviewed to document use of elastics, length of treatment time, and age when treatment was initiated. Incisor length measurements were compared using a two-sample t-test with p set to 0.05. Using a Chi-square test for RxC contingency tables with p set to 0.05, the percentage change in incisor length was compared categorically: no resorption, mild (1-10% decreased length), moderate (10-30%), and severe (>30%). Treatment factors and changes in tooth position were analyzed using multiple linear regression analysis. Results showed there was a significant difference in the amount of incisor root resorption in patients with an open bite (2.26 mm; +/- SD 1.76 mm) compared to patients with a normal overbite (0.93 mm; +/- SD 1.16 mm, p = 0.001). The Chi-square test showed that the distribution of root resorption categories was significantly different for the two groups (p = 0.01). Multiple linear regression analysis indicated that the presence of an open bite and longer central incisor length s were predictive of increased root resorption. Of the treatment factors analyzed, none were identified that increased the risk for root resorption. In conclusion, this study suggests individuals with anterior open bite malocclusion that undergo orthodontic treatment have more risk for root resorption, but treatment factors associated with the increased risk could not be elucidated.

It is common practice for orthodontists to use panoramic images to assess root parallelism before, during and after orthodontic treatment. The purpose of this laboratory study was to examine the effects of changes in buccolingual root torque on mesiodistal root angulation imaged on panoramic radiographs. Teeth investigated were maxillary lateral incisors and second premolars, and mandibular first and second premolars. Using a transparent typodont model with anatomically correct teeth (typical crown and root anatomy), root torque was progressively added to the teeth in 5º increments, totaling up to 15º of torque in each direction. Standardized digital photographs and panoramic radiographs were made after each adjustment. The photographs made from a facial perspective documented that the roots remained parallel. With each increment of change in root torque, measurement of the root apex position in a mesiodistal direction was made on the panoramic radiograph. Results showed that when torque was changed, distortion inherent to the panoramic radiograph created the appearance of mesial or distal movement of the root apices. For the premolars, adding lingual root torque (LRT) resulted in apparent increases in mesial root tip (e.g., with 15º LRT, the root tips moved mesially 1.9 to 2.8 mm) while adding buccal root torque (BRT) resulted in apparent increases in distal root tip (e.g., with 15º BRT, the root tips moved distally 2.8 to 4.4 mm). For the lateral incisors, changes in torque had a less pronounced effect on apparent root tip (e.g., with 15° LRT, the root tips moved mesially 1.2 mm; with 15° BRT, the root tips moved distally 0.17 mm). The results show that changes in root torque predictably affect the appearance of root tip on panoramic radiographs. The findings suggest that if roots appear incorrectly aligned on panoramic images, the teeth in question should be carefully examined clinically (or with periapical images) to differentiate the need for appliance adjustments that alter root tip versus root torque.

For orthodontic treatment of certain malocclusions, Invisalign (Align Technology, Inc., Santa Clara, CA) offers an alternative to fixed appliances. The purpose of this study was to evaluate the treatment outcome of patients treated with Invisalign aligners using the American Board of Orthodontics Objective Grading System (OGS) for dental casts. The dental cast component of the OGS scores 7 categories for discrepancies from ideal related to tooth alignment and occlusion. Depending on the magnitude of the discrepancy, up to two points can be scored in each category, thus the fewer the points scored, the more ideal the result. A total of 135 cases treated with the Invisalign system and having pre- and post-treatment records were identified from 7 orthodontic private practices. If cases met the selection criteria, the dental cast component of the OGS was performed on both the pre- and post-treatment casts. The selection criteria was: (1) having no missing teeth (other than previous bicuspid extractions), (2) having been treated exclusively with Invisalign, and (3) available pre- and post-treatment models of diagnostic quality. Pre- and post-treatment OGS scores were compared and the differences were analyzed using paired t-tests. Results showed that of the 135 cases, 65 (48%) met the inclusion criteria. These were evaluated using the OGS. For the cases not included in the OGS analysis, 33 (24% of the 135) required the use of fixed appliances or spring aligners to finish the treatment , 25 (19%) had poor or missing records, and 12 (9%) had missing teeth. For the 65 cases analyzed, the mean patient age was 31.1 (+/- standard deviation 12.5) years, the duration of treatment was 12.5 (+/- 4.1) months, and the patients wore 17.9 (+/- 7.3) upper and 17.7 (+/- 6.9) lower Invisalign aligners. The pre-treatment total OGS score (dental component only) was 47.0 (+/- 16.5) and the post-treatment OGS score was 36.4 (+/- 16.0; p < 0.001). Comparison of the categories in the OGS showed that significant improvements occurred in tooth alignment (pre-treatment mean: 21.5 points vs. post-treatment: 7.0 points (p < 0.001), buccolingual inclination (4.7 vs. 4.1 points, p= 0.001), and interproximal spaces (1.3 vs. 0.0 points; p <0.05). On the other hand, there was a negative change in posterior occlusal contacts (4.0 vs. 10.3 points; p < 0.001). The remaining categories including marginal ridge relationships, occlusal relationships (e.g, posterior interdigitation), and overjet (anterior and posterior), showed no significant change. Thus, for this sample treated with the Invisalign system the greatest positive change as measured by the OGS point system was in the alignment of teeth, followed by closure of interproximal space. An undesirable change was found with a decline in posterior occlusal contacts. The score for the latter category had a negative impact on the overall changes as measured with the OGS method. In conclusion, results of the OGS show that treatment with Invisalign aligners had adverse effects on posterior occlusal contacts and positive effects on tooth alignment, buccolingual inclination and interproximal spaces.